Apparatus and method for isolating in-channel FM antennas sharing common aperture space

Abstract

Each of a pair of antennas for broadcasting has multiple elements arranged vertically on the same tower. The antennas transmit circularly polarized signals of opposite polarization. The opposite circular polarization of the radiated signals increases their mutual isolation and permits broadcast of conventional FM-band signals and digital FM at the same frequency. The polarization technique allows the elements of the two antennas to share an aperture without degradation of function.

1. An enhanced-isolation shared-aperture digital and analog FM antenna pair comprising:
 a first independent FM transmitting antenna located in an aperture space on a tower, wherein the first antenna has a plurality of elements and has a first orientation of polarization; and  
 a second independent FM transmitting antenna interleaved principally within the same aperture space of the first antenna, wherein the second antenna has a plurality of elements and has a second orientation of polarization different from the orientation of polarization of the first antenna, so as to increase signal isolation between the second antenna and the first antenna.  
 
   
   
     2. The antenna pair of  claim 1 , wherein the first antenna comprises a first plurality of individually driven first elements interleaved vertically along the tower so that the first elements, when driven, radiate a single first polarized transmitted signal. 
   
   
     3. The antenna pair of  claim 1 , wherein the second antenna comprises a second plurality of individually driven second elements interleaved vertically along the tower so that the second elements, when driven, radiate a single second polarized transmitted signal. 
   
   
     4. The antenna pair of  claim 1 , wherein the elements comprising the first antenna have symmetrical and opposite physical arrangement to the elements comprising the second antenna. 
   
   
     5. The antenna pair of  claim 1 , wherein the elements of the first of the two antennas, when driven, radiate a right-hand circularly polarized signal, and the elements of the second antenna, when driven, radiate a left-hand circularly polarized signal. 
   
   
     6. The antenna pair of  claim 4 , wherein the elements of the first of the two antennas, when driven, radiate a right-hand circularly polarized signal, and the elements of the second antenna, when driven, radiate a left-hand circularly polarized signal. 
   
   
     7. The antenna pair of  claim 1 , wherein the first elements comprising the first antenna are interleaved with the second elements comprising the second antenna. 
   
   
     8. The antenna pair of  claim 1 , further comprising:
 a first power divider and associated first coaxial lines that establish the phase relationship between the first elements comprising the first antenna of the pair; and  
 a second power divider and associated second coaxial lines that establish the phase relationship between the second elements comprising the second antenna of the pair.  
 
   
   
     9. The antenna pair of  claim 1 , further comprising:
 an analog coaxial feed line feeding said analog antenna;  
 a first analog-signal tee junction in said analog-signal coaxial feed line distributing analog signal energy among the elements comprising said analog antenna;  
 a first analog-signal subordinate coaxial line coupling analog-signal energy from said first analog-signal tee junction to a first analog antenna element; and  
 a second analog-signal subordinate coaxial line coupling analog-signal energy from said first tee junction to a second analog antenna element.  
 
   
   
     10. The antenna pair of  claim 1 , further comprising:
 a digital coaxial feed line feeding said digital antenna;  
 a first digital-signal tee junction in said digital-signal coaxial feed line distributing digital signal energy among the elements comprising said digital antenna;  
 a first digital-signal subordinate coaxial line coupling digital-signal energy from said first digital-signal tee junction to a first digital antenna element; and  
 a second digital-signal subordinate coaxial line coupling digital-signal energy from said first digital-signal tee junction to a second digital antenna element.  
 
   
   
     11. The antenna pair of  claim 1 , wherein the centers of radiation of the antennas comprising the pair are separated by a distance approximating one-half wavelength of the center frequency of the broadcast channel to which the antennas are tuned. 
   
   
     12. The antenna pair of  claim 1 , wherein the centers of radiation of the antennas comprising the pair are separated by a distance approximating one half of the center-to-center distance between the elements comprising the first antenna of the pair. 
   
   
     13. The antenna pair of  claim 1 , wherein the center of radiation of the second antenna is located within the aperture of the first antenna. 
   
   
     14. The antenna pair of  claim 1 , wherein the elements comprising the first antenna and the second antenna are polarized ring style antenna elements. 
   
   
     15. The antenna pair of  claim 1 , wherein the spacing between successive elements of the first antenna is one wavelength of the center frequency of the broadcast channel to which the antennas are tuned. 
   
   
     16. The antenna pair of  claim 1 , wherein the spacing between successive elements of the first antenna is a uniform value different from one wavelength of the center frequency of the broadcast channel to which the antennas are tuned. 
   
   
     17. The antenna pair of  claim 1 , wherein the spacing between successive elements of the second antenna is the same as the spacing between successive elements of the first antenna. 
   
   
     18. The antenna pair of  claim 1 , wherein the distance between each element of the first antenna and any elements of the second antenna proximal thereto is uniform. 
   
   
     19. The antenna pair of  claim 1 , wherein all of the elements comprising the first and second antennas are attached to a common supporting tower with substantially identical orientation with respect to the cross section of the tower. 
   
   
     20. An apparatus for transmitting digital and analog FM radio signals from a common aperture space, comprised of:
 means for radiating a first FM signal with a first polarization; and  
 means for radiating a second FM signal with a second polarization different from that of the first signal, wherein the first and second radiating means are interleaved within the same aperture space and the different polarizations increases signal isolation between the radiating means.  
 
   
   
     21. The transmitting apparatus of  claim 20 , further comprising:
 means for accepting a first broadcast-level FM radio signal from a first transmission line; and  
 means for distributing the energy of the first broadcast-level FM radio signal among multiple transmitting elements to create a first polarized transmission.  
 
   
   
     22. The transmitting apparatus of  claim 20 , further comprising:
 means for accepting a second broadcast-level signal from a second transmission line; and  
 means for distributing the energy of the second broadcast-level signal among multiple transmitting elements to create a second polarized transmission with polarization different from that of the first signal.  
 
   
   
     23. A method for simulating analog and digital FM broadcasts from a single aperture space comprising the steps of:
 vertically interleaving a second antenna within a same aperture space of a first antenna;  
 driving the first antenna with a first polarized FM signal; and  
 driving the second antenna with a second polarized FM signal, where the polarizations of the two signals are different, so as to increase signal isolation between the antennas.  
 
   
   
     24. The method of  claim 23 , further comprising the steps of:
 accepting a first broadcast-level signal from a first transmission line; and  
 distributing the energy of the first broadcast-level signal among multiple transmitting elements to create a first polarized transmission.  
 
   
   
     25. The method of  claim 23 , further comprising the steps of:
 accepting a second broadcast-level signal from a second transmission line; and distributing the energy of the second broadcast-level signal among multiple transmitting elements to create a second polarized transmission with polarization different from that of the first signal.